DISCUSSION ON HYDROGEN TRANSPORT 41 



pyridine enzyme, not much is definitely known about such action 

 of flavoproteins. One example, however, is the dismutation of pyruvic 

 acid (Lipmann), in which a flavin component was shown to par- 

 ticipate in a coupled oxidoreduction. Here the flavin mediates 

 between two enzyme systems of different types, i.e., between lactic 

 and pyruvic dehydrogenase: 



( 1 ) pyruvate — > lactate 

 2HT (flavin) 



pyruvate phosphate — > acetylphosphate + CO2 



The presence of a flavoprotein in yeast, which catalyzes the reduc- 

 tion of fumarate (Fischer), suggests the mediator function of such a 

 flavoprotein between pyridine enzymes and fumarate. For example, 

 in the dismutation of fumaric acid (Green) electron transfer pre- 

 sumably occurs between malic dehydrogenase, a pyridine enzyme, 

 and succinic dehydrogenase, a non-pyridine enzyme: 



( 2 ) fumarate — » succinate 

 2H T (flavin) 



fumarate «^ malate -^ oxalacetate 



It is probably accidental that in both examples for anaerobic 

 flavin mediation, the coupled oxidoreduction is a so-called dismu- 

 tation. This might serve to show that in most enzymatic dismutations 

 the underlying reaction is, in fact, an oxidoreduction between two 

 enzyme systems catalyzing two fundamentally different reactions. 

 But in dismutation the metabolic substrates for both enzyme sys- 

 tems derive from the same compound. In the cases discussed, the 

 oxidant in the oxidation-reduction reaction is the compound proper, 

 pyruvate or fumarate, and the reductant is a transformation product 

 of the added compound. Oxidant and reductant belong, respectively, 

 to two different oxidation-reduction systems with widely different 

 oxidation-reduction potentials. In dismutation (reaction 1) the lac- 

 tate-pyruvate system of EJ —0.18 volts reacts with the pyruvate... 

 HX —acetate +CO2 system of Eo' below —0.4 volts. In the sec- 

 ond reaction the succinate-fumarate system of Eq' 0.0 volts reacts 

 with the malate-oxalacetate system of Eo' —0.17 volts. These large 

 energy differences between the reacting oxidation-reduction sys- 

 tems explain why dismutation occurs. 



REFERENCES 



1. Dixon, Malcolm, and Lutwak-Mann, Cecilia, Biochem. J., 31, 1347 

 (1937). 



2. Green, Davto Ezra, Biochem. J., 28, 1559 (1934). 



